As is well-known, carbon artifacts have been made by pyrolyzing a wide variety of organic materials. Indeed, one carbon artifact of particularly important commercial interest today is carbon fiber. Hence, specific reference is made herein to carbon fiber technology. Nevertheless, it should be appreciated that this invention has applicability to carbon artifact manufacturing generally, and most particularly, to the production of shaped carbon articles in the form of filaments, yarns, films, ribbons, sheets and the like.
The use of carbon fibers for reinforcing plastic and metal matrices has gained considerable commercial acceptance. The exceptional properties of these reinforcing composite materials, such as their high strength to weight ratio, clearly offset their high preparation costs. It is generally accepted that large scale use of carbon fibers as a reinforcing material would gain even greater acceptance in the marketplace, if the costs of the fibers could be substantially reduced. Thus, formation of carbon fibers for relatively inexpensive carbonaceous pitches has received considerable attention in recent years.
Many materials containing polycondensed aromatics can be converted at early stages of carbonization to a structurally ordered optically anisotropic spherical liquid crystal called mesophase. The presence of this ordered structure prior to carbonization is considered to be fundamental in obtaining a high quality carbon artifact. Thus, one of the first requirements of a feedstock material suitable for carbon artifact manufacture, and particularly for carbon fiber production, is its ability to be converted to a highly optically anisotropic material.
In addition, suitable feedstocks for carbon artifacts manufacture, and in particular carbon fiber manufacture, should have relatively low softening points and sufficient viscosity suitable for shaping and spinning into desirable articles and fibers.
Unfortunately, many carbonaceous pitches have relatively high softening points. Indeed, incipient coking frequently occurs in such materials at temperatures where they have sufficient viscosity for spinning. The presence of coke, infusible materials, and/or high softening point components are detrimental to the fiber making process.
As is well-known, pitches have been prepared from the total tars obtained from steam cracking of gas oil or naphtha. In this regard, see, for example, U.S. Pat. Nos. 3,721,658 and 4,086,156.
Steam cracker tar, like other heavy aromatics, is composed of a complex mixture of alkyl-substituted polycondensed aromatics. The chemical structure, molecular weight and aromatic ring distribution can be determined quantitatively using advanced analytical methods such as carbon and proton nuclear resonance spectroscopy or mass spectrometry.
Steam cracker tar, like other heavy aromatics such as coal tars and tars from catalytic or fluid cracking, is composed of two major parts: (1) a low molecular oil; and (2) a high molecular weight fraction called asphaltene, which is insoluble in a paraffinic solvent. The asphaltene in steam cracker tar varies from 10-30 wt % depending on the type of feedstock being introduced into the cracker, the design of the cracker and the severity of the cracking.
Asphaltenes can be determined quantitatively in steam cracker tar using n-heptane.
The two aforementioned parts of steam cracker tar, i.e., the oil and the asphaltene, vary significantly in their chemical composition, molecular weight, melting characteristics and most importantly their coking characteristics.
The asphaltene presence in the steam cracker tar tends to be detrimental to carbon artifact manufacture, because it produces coke in the pitch and more importantly it does not provide a pitch with a high liquid crystal content; i.e., it severely limits the composition of the pitch.